1. Field of the Invention
The present invention relates generally to an apparatus for reproducing a video signal recorded in parallel skewed tracks on a magnetic recording medium in a helical scan-type video tape recorder, and is directed more particularly to an apparatus for correcting for errors resulting from a tape transport speed during reproduction which is different from the tape transport speed employed during recording.
2. Description of the Prior Art
The prior art contains examples of attempts to correct mistracking due to reproduction at a different tape transport speed from that used during recording.
In an attempt to increase the recording density, video signals are recorded in adjacent skewed tracks without unrecorded guardbands therebetween. Consequently, if no measures were taken to prevent it, while reproducing a signal in one track, a reproducing head would inevitably reproduce a crosstalk signal from one or both adjacent record tracks. In order to reduce or eliminate such crosstalk which interferes with reproduced signals, the prior art employs first and second recording heads alternately recording in adjacent parallel tracks. The azimuth angles of the recording gaps in the two recording heads are made different so that the magnetic domains created by recording in adjacent tracks are skewed with respect to each other by an angle of suitably 60.degree.-90.degree.. Consequently, when reproduction is performed with first and second reproducing heads having azimuth angles corresponding to the azimuth angles of the first and second recording heads, crosstalk components from adjacent tracks, which have their magnetic domains skewed relative to the azimuth angles of the operative head, are substantially attenuated.
When a video signal recorded on the tape is reproduced at a tape transport speed different from that used during recording, the path of the reproducing heads is additionally influenced by the difference in tape transport speed. The resulting additional skew component causes each reproducing head to follow a path which deviates from the direction along a record track. During still reproduction, for example, the path may begin in coincidence with a first track and end in coincidence with a second adjacent track. Assuming that the recorded azimuth angle of the first track matches the azimuth angle of a first reproducing head, the reproduced signal would begin at full strength during scanning of the first track by the first reproducing head and, as such head passes over to the second adjacent track recorded with a different azimuth angle, the reproduced signal would be substantially attenuated due to azimuth loss. In the case of a system in which a complete television field is recorded on a single track, this would result in the display of a field containing a useful picture only in the upper portion of a screen and would have a poor signal to noise ratio in the lower portion. If the same path were to be followed by the second reproducing head to reproduce a second field interlaced with the first field, the first portion of the track would have a poor signal to noise ratio due to azimuth loss, whereas the later portion of the track would produce a useful signal. Thus, the upper portion of the second field would be degraded but its lower portion would be at full strength. Since both interlaced fields contain significant loss, the quality of the entire picture is seriously degraded.
The above-described problem is addressed in auto-tracking systems in the prior art by mounting the reproducing heads on deflectable members which may be electrically biased or shifted perpendicular to the scanning direction to produce compensating displacement of the reproducing heads which force them to accurately trace the recorded track. The deflectable members may be, for example, bi-morph leaves which are made of, for example, two laminated piezo-ceramic leaves. An example of the use of such a deflectable member is disclosed in, for example, U.S. patent application Ser. No. 731,763 filed Oct. 12, 1976, now abandoned, and assigned to the assignee of the present invention. In such prior art tracking systems for use with skewed tracks recorded on magnetic tape, the reproducing head is moved or vibrated at right angles to the head tracing or scanning direction. This compensates for the additional skew component resulting from the change in tape transport speed. However, an additional error component is produced in the along-track, or time base, direction due to the difference in recording and reproducing speeds. During one scanning period (normally 1/60 of a second), the distance along the tape scanned by a reproducing head during still reproduction is greater than the distance scanned during recording. Therefore, if scanning by the first head is begun on a track at the point where the recorded material commences in such track, the head will scan an unrecorded area on the tape at the end of such track when moved at right angles to the scanning direction for accurate tracking. The second recording head, being similarly moved at right angles to the scanning direction, begins reproducing on unrecorded tape before it reaches the recorded track. This along-track, or time base error is not an integral multiple of a horizontal interval, but instead is a fraction 1/n of a horizontal interval. The resulting disruption in the timing of horizontal synchronizing pulses during reproduction unlocks horizontal synchronization and produces a skew in the reproduced picture.
In the converse case of reproduction at a tape transport speed greater than the tape transport speed during recording, the along-track error is an effective shortening of the scanning path during the scanning period, and again produces disruption in horizontal synchronization and skewing of the picture.
Time base error correction systems have been applied to linear recording devices in the prior art. For example, U.S. Pat. No. 3,787,616, issued Jan. 22, 1974, discloses a time base error correction system for use in a linear magnetic tape recording or playback apparatus and which employs a bi-morph leaf to displace an otherwise stationary recording or playback head in the along-track direction as the tape moves past it according to the coincidence between detected horizontal synchronizing pulses and the output of a reference oscillator.
A time base correction system for linear tape recording is also disclosed in U.S. Pat. No. 3,526,726, issued Sept. 1, 1970. This patent discloses time base error and guide error correction in a linear parallel multiple track tape recorder device. In this disclosure, a multiple parallel track tape is drawn past a substantially stationary record-reproduce head which provides means for sensing and correcting for time displacement between a track and a reference signal as well as for sensing and correcting relative time displacement between tracks of the same tape.
The problems to be solved by the present invention arise when skewed recording tracks are employed and such problems are neither recognized nor addressed in the two above cited prior patents which deal with linear recording apparatus.